Add gem packaging tasks to Rakefile

This makes it slightly easier for us to build a new gem, as now we can
just run:

    rake build

...instead of:

    gem build overcommit.gemspec

Change-Id: I69fd23ff1dbae15c8232d13995921c0bee68e5b7
Reviewed-on: http://gerrit.causes.com/45335
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>